Function write_new_file_with 

fn write_new_file_with(
    path: &Path,
    contents: &[u8],
    before_persist: impl FnOnce() -> Result<()>,
) -> Result<()>

A lock-free, durable, no-clobber file create (SP2 design §4.4). There is no sweep of leftover temp files: without a lock a sweep could delete another launch’s live temp file.